The present invention relates to systems for supplying radio frequency (RF) power to a drive electrode, particularly in a plasma reactor.
Manufacturers of equipment used by integrated circuit (IC) fabricators face a constant demand to improve the performance of their equipment. One particular type of equipment that is widely used is the plasma reactor, which is used to remove material from, or deposit material on, a semiconductor wafer. The performance of such equipment depends on coupling energy efficiently to electrodes in the reactor.
One of the key factors that determine the yield and overall quality of an IC is the uniformity of processes, such as the etch rate in vias and trenches, on the wafer. In plasma reactors, the uniformity of a process across the surface of a wafer is governed by the design of the overall system. Uniformity is typically compromised due to generation within the plasma of harmonics of the RF drive frequency because the plasma is electrically a non-linear medium, i.e., the impedance of the plasma varies as a non-linear function of electric field intensity. Significant levels of power can be generated at harmonics as high as the twentieth. These harmonics result in standing RF waves at the harmonic frequencies on the drive electrode and in the RF feed system between the driving electrode and the RE matching network, which in turn leads to plasma non-uniformities and consequent process non-uniformities across the surface of the wafer.
In a capacitively coupled plasma reactor, power from an RF power source is supplied to a drive electrode via an impedance match network. Such a reactor may have a plurality of RF drive electrodes and RF feed systems. The discussion that follows is applicable to each RF drive electrode, but for simplicity only one such RF drive system is considered below. Similarly, the invention is applicable to each RF drive system of a multi-electrode or segmented electrode drive structure. The harmonic components are generated within the plasma and propagate from the drive electrode back to the match network and the RF power source. The harmonic components are then typically reflected back to the drive electrode and standing waves at the harmonic frequencies are thereby created.
The invention provides an improved electrode assembly for supplying RF power from an RF power source having an output impedance to an electrically non-linear medium, the assembly including: a drive electrode in communication with the medium and forming with the medium a load impedance; and an impedance match network connected between the RF power source and the drive electrode for matching the output impedance of the RF source to the load impedance and having an output component connected directly to the drive electrode.
The electrode assembly according to the invention can further include a body of RF energy absorbing material disposed between the match network and the drive electrode, the absorbing material having a frequency dependent energy absorption characteristic which increases with frequency. The provision of a body of RF energy absorbing material will improve process uniformity.